1. Field of the Invention
The invention relates to a microprobe useful for assessing auditory function by enabling clinical and intraoperative measurements of blood flow, particularly cochlear blood flow and neural compound action potentials, particularly of the cochlea and auditory (vestibulocochlear) nerve, also known as cranial nerve VIII.
2. Description of Related Art
Interruption of cochlear blood flow and damage to the auditory nerve have been implicated as the primary causes of sensory hearing loss that may occur during acoustic neuroma tumor removal. Neurosensory monitoring involving electrophysiological signals and continuous blood flow measurements have been proposed to save the hearing of patients undergoing such procedures. The most commonly monitored electrophysiological signals are the auditory brainstem response (ABR) and the compound action potential (CAP) of cranial nerve VIII. Each of these traditionally targeted signal sources have drawbacks, making theses sources less than optimal for mitigating hearing loss.
Auditory brainstem response is the most easily applied procedure and is noninvasive, yet suffers from low signal to noise ratios (SNR). As a result, extensive averaging must be performed to generate usable data.
Measurement of compound action potentials provides better signal to noise ratios than may be obtained with auditory brainstem response. However, compound action potential monitoring requires invasive techniques for optimal recording conditions, and requires at least some averaging to obtain recognizable responses. Although both auditory brainstem response and compound action potentials provide critical information about the transfer of auditory information from the cochlea to the auditory nerve, they are not the best methods for monitoring cochlear ischemia due to delays in changes in the measures after alterations in cochlear blood flow. For example, due to metabolic reserve, changes in electrophysiological activity after an interruption of blood flow in the internal auditory artery take about 20-60 seconds.
Monitoring cochlear blood flow with laser-Doppler measurements has the potential to overcome some of the drawbacks of electrophysiological monitoring. Since it directly monitors blood flow to the cochlea, laser-Doppler does not succumb to metabolic reserve or prolonged signal averaging problems, and has been shown in animal models to follow changes in cochlear blood flow in near real time. The rapid feedback from laser-Doppler can provide surgeons with timely information regarding the effect of surgical maneuvers and vasospasm on cochlear blood flow, with the potential to reverse both cochlear ischemia and adverse hearing outcome.
Electrocochleography (EcochG) is a measure of the most peripheral of the neuroelectric auditory-evoked responses. The compound action potential component of the EcochG response represents the same activity as Wave I of the auditory brainstem response, and is the most useful for intraoperative monitoring. The EcochG has the advantage of being a near-field recording and as such requires fewer averages and less time to obtain a response. Detailed descriptions of stimulus and recording parameters are known in the art. The EcochG can be recorded from the external auditory canal or within the middle ear (transtympanic). Previous investigators have compared the time and number of sweeps required to obtain a response using both techniques. EcochGs recorded within the middle ear showed improved signal to noise ratios resulting in waveforms obtained with fewer sweeps over a shorter period of time. Positioning of the recording electrode at the round window provides the most robust response. The surface or subdermal reference electrode is placed in the midline between the vertex and forehead. Ground electrodes can be located near the recording electrodes at a convenient spot (e.g. ipsilateral shoulder or contralateral forehead).
The stimulus should be a broad band rarefaction click of high intensity (85 to 95 dBnHL) with a rate of 21.1 sec. An impedance of as high as 100 kxcexa9 may be acceptable in a transtympanic (middle ear round window) montage as compared to the need for extremely low impedance of less than 5Kxcexa9 required for ear canal electrodes.
Adequate preoperative baseline responses must be obtained against which to judge changes observed during the procedure. Significant changes in compound action potentials indicate, and can be used to differentiate between, cochlear and neural injury. Twenty seconds or more must elapse before changes in the EcochG response can be detected. This delay presumably occurs as a result of metabolic reserves which sustain cochlear function until their depletion from prolonged ischemia causes failure of electrophysiologic activity.
When a parallel laser beam is incident on a medium containing randomly distributed particles, a fraction of the incident power is absorbed by the particles and a fraction is scattered, in theory at all angles. The angular intensity distribution of the scattered laser radiation depends mostly on the wavelength and geometry of the incident beam and on the distribution and size of the scattering particles. When the scattering particles are stationary, the scattering angle for each scattering event, and thus the spatial distribution of intensity, is stationary as well.
When the scattering particles move at any given velocity, the scattering angle varies with the particle velocity and with the angle between the direction of incidence and the direction of movement of the particle. As a result, the scattered light intensity measured in any direction is no longer stationary. The scattered signal contains a broad spectrum of frequencies which is a function, among others, of the particle velocity distribution and of the direction of the particles. A frequency analysis of the scattered intensity signal in a given direction provides information on the velocities of the particles. Because the frequency shifts when the speed or direction of the particles varies, this optical technique for flow measurement is called laser Doppler velocimetry.
Several optical configurations have been used for laser Doppler velocimetry. Laser Doppler velocimetry has been used in biology and medicine mostly as a noninvasive diagnostic tool to characterize blood flow in vitro and in vivo in the eye or percutaneous tissue.
The feasibility of using laser-Doppler velocimetry for in situ in vivo measurement of blood flow was demonstrated by several investigators by using an optical fiber design. In fiber optic laser Doppler velocimeters, the incident light signal is delivered to the measured tissue volume through a single multimode optical fiber (excitation fiber) with a diameter of 50 to 100 microns. The light scattered back by the tissue is collected either with the same optical fiber, or with one or two separate optical fibers (collecting fibers) located next to the excitation fiber. The light transmitted through the collecting fibers is sent to separate photodetectors connected to the signal processing unit. The main advantage of fiber optic laser Doppler velocimetry is that the excitation and collecting fibers can be integrated into a miniature hand held probe for minimally invasive in situ measurements of blood flow. The design of the probe can be adapted for different types of measurement conditions, including endoscopic measurements or measurements at 90 degrees.
The blood supply to the cochlea in humans and animals is provided by an end-artery branch of the internal auditory artery. Abolition of this blood supply results in a complete loss of auditory function in animals. Measurement of cochlear blood flow has been of experimental interest because cochlear ischemia is assumed to be one of the principal causes of certain types of presbycusis and for the many cases of sudden idiopathic sensorineural hearing loss (SNHL). Vascular compromise of the cochlea is commonly thought to cause some forms of sound-induced acoustic trauma. Additionally, some of the hearing loss encountered in patients with acoustic neuromas may also be caused by cochlear ischemia. In addition to the vascular-related effects of disease states on inner ear function, the tenuous nature of the cochlear blood supply becomes important during certain surgical procedures performed around the internal auditory canal/CPA (cerbellopontine angle) region and posterior fossa where post-operative hearing loss is thought to be due to compromise of the internal auditory artery.
Shortly after the initial reports of laser-Doppler measurements of blood flow in general, successful measures of cochlear blood flow using laser-Doppler were accomplished. Use of laser-Doppler systems makes it possible to routinely measure cochlear blood flow in experimental animal models, including the guinea pig. In such small laboratory species as gerbils and guinea pigs, the laser beam of these instruments can be easily directed toward the capillary bed if the stria vasculairs due to the translucency of the cochlea""s bony capsule. In other animal species, such as rabbits and humans, in which the cochlea is embedded in dense temporal bone, the laser beam is greatly attenuated, thereby, adversely compromising its measurement capabilities.
In humans, to date, only a few experimental findings on laser-Doppler measures of cochlear blood flow are available. For example, some reports indicate that cochlear blood flow is measurable by positioning a laser-Doppler probe directly on the promontory cochlea. On this position, a transient decrease in cochlear blood flow was noted during breath holding. Those reports also indicate that the thick cochlear bone of the human attenuates the laser beam up to four times more than does the more translucent otic capsule of the guinea pig. Due to this reduced sensitivity, some investigators have concluded that the mucosal vasculature of the promontory contacting the probe likely contaminated some cochlear blood flow readings.
In a later study, other investigators also placed a laser-Doppler probe on the promontory of a few patients in order to measure cochlear blood flow under several conditions. Specifically, they showed that cochlear blood flow changed more dramatically during irrigation of the external ear canal with either warm or cold water than during electrical stimulation of the cochlea using an electrode on the round window membrane. However, the authors cautioned that movement related artifacts, which occurred mostly during the irrigation procedure, could have also contributed to the robust changes in cochlear blood flow they observed. Nonetheless, these authors were confident that their careful measurements avoided such contamination of the data.
In both studies noted above, cochlear function was not monitored simultaneously with cochlear blood flow measurements, and, for obvious ethical reasons, deliberate induction of transient ischemia in patients was not performed to test the validity of laser-Doppler cochlear blood flow changes. One opportunity to assess the sensitivity of laser-Doppler measures of cochlear blood flow in human cases of cochlear ischemia would be to monitor the blood flow to the cochlea during surgery to remove vestibular schwannomas, when, at time, an unplanned interruption of blood flow can occur. However, no such reports are currently known, mostly due to the fact that until recently, stable laser-Doppler cochlear blood flow baseline values have been difficult to obtain throughout the lengthy surgical procedures. The opacity of the cochlear bone over the promontory region likely precluded any sensitive laser-Doppler measurements.
Therefore, a need exists for improved means of measuring signals indicative of auditory function to reduce hearing loss during surgery and diagnose hearing loss in clinical settings.
A first embodiment of the invention is an integrated otic microprobe for atraumatically monitoring auditory function in a patient. The microprobe comprises a fiber optic laser Doppler flowmetry probe which measures blood flow and velocity, an electrocochleography electrode which measures neural compound action potentials, and a cap encompassing a tip of the fiber optic laser Doppler flowmetry probe. Also included in the microprobe is at least one irrigation lumen, at least one aspiration lumen, and a means for conducting an electrocochlear signal from the laser Doppler flowmetry probe tip encompassed by the cap to a computerized data monitoring unit. The integrated otic microprobe may further comprise a flexible endoscope, in which case the microprobe has a diameter of less than 2 mm.
In another embodiment, the integrated otic microprobe according to the invention comprises a cap that fits within a middle ear round window. The integrated otic microprobe measures substantially cochlear blood flow via the fiber optic laser Doppler flowmetry probe. The integrated otic microprobe also measures substantially compound action potentials of cranial nerve VIII via the electrocochleography electrode. The termxe2x80x9cmeasures substantiallyxe2x80x9d is defined to mean accurately detecting data signals in near real-time.
In yet another embodiment, the invention comprises a system for intraoperatively monitoring auditory function. The system comprises a hand held device for insertion into an ear. This hand held device houses an integrated, multi-membered otic fiber optic laser microprobe. Also included in the system are reference and ground electrodes, a diode laser excitation source, and a computerized data monitoring unit. An electronic control unit connects a first member of the fiber optic laser microprobe to the computerized data monitoring unit. The system also comprises a sound generator. A high input impedance bioamplifier of the system connects a second member of the fiber optic laser microprobe, the reference electrode and the ground electrode to the computerized data monitoring unit. This integrated otic fiber optic laser microprobe of the system comprises a first member comprising a fiber optic laser Doppler flowmetry probe which measures blood flow and velocity; a second member comprising an electrocochleography electrode which measures neural compound action potentials; a cap encompassing a tip of the fiber optic laser Doppler flowmetry probe; at least one irrigation lumen; at least one aspiration lumen; and a means for conducting an electrocochlear signal from the laser Doppler flowmetry probe tip encompassed by the cap to the computerized data monitoring unit. The bioamplifier amplifies, filters and transmits electrocochlear electrode responses to the computerized data monitoring unit. The laser Doppler flowmetry probe comprises at least one emission fiber and one or more sensing fibers. Said at least one emission fiber and two or more sensing fibers are housed within a needle probe. The reference electrode detects auditory brainstem responses.
In the system of the invention, the hand held device and cap preferably comprise a medical grade polymer, preferably and elastomer. Examples of suitable elastomers include polydimethylsiloxane, urethane, silicone or other flexible silicone-based polymers.
In another embodiment, the system of the invention comprises a means for conducting an electrocochlear signal. The signaling means may be a member selected from the group consisting of a metal tube surrounding the laser Doppler flowmetry probe tip and a platinum wire in direct contact with an otic round window. Other similar types of signal conducting means, such as wires and metal surfaces may be used.
Still another embodiment of the invention is a system in which at least one irrigation lumen and at least one aspiration lumen irrigate a middle ear round window cavity through the cap encompassing the tip of the fiber optic laser Doppler flowmetry probe. In such a system, the at least one irrigation lumen and the at least one aspiration lumen may be connected to a peristaltic pump.
In yet another embodiment of the invention, the system further comprises a channel for a flexible endoscope comprising a fiber bundle for illumination, and a fiber optic imaging system including an objective lens and a flexible imaging bundle. In such a system, the flexible endoscope preferably has an overall diameter of less than 0.8 mm.
A preferred embodiment of the invention is a method of atraumatically monitoring auditory function in a patient. This method comprises inserting into a patient""s ear a microprobe comprising a fiber optic laser Doppler flowmetry probe which measures blood flow and velocity; an electrocochleography electrode which measures neural compound action potentials; a cap encompassing a tip of the fiber optic laser Doppler flowmetry probe; at least one irrigation lumen; at least one aspiration lumen; and a means for conducting an electrocochlear signal from the laser Doppler flowmetry probe tip encompassed by the cap to the computerized data monitoring unit. Upon insertion into the patient""s ear canal, the cap of the microprobe is positioned against the round window membrane of the middle ear. Positioning the microprobe in this manner enables measuring cochlear blood flow via the laser Doppler flowmetry probe and measuring compound action potentials of cranial nerve VIII via the electrocochleography electrode following stimulation of an auditory response with a noise at a predetermined frequency and loudness. Auditory function may then be assessed by comparing measured blood flow and compound action potential values against baseline blood flow and compound action potential values taken prior to the auditory response is stimulated.